1. Field of the Invention
The present invention relates to an image reading apparatus for reading an image in the unit of each pixel, and more particularly to an image reading apparatus provided with an illuminating light source.
2. Description of the Related Background Art
There is already known an image reading apparatus, for use in a facsimile apparatus or a digital copying machine, in which an original image is illuminated by a light source such as a fluorescent lamp and the intensity of the light reflected from said original image is detected for example by a CCD image sensor to obtain a photoelectrically converted electric signal.
In such apparatus, the intensity of the illuminating light source affects the resulting image signal and has therefore to be regulated to an optimum value.
The assignee of the invention has already proposed, in U.S. Pat. No. 4,677,287, U.S. Pat. No. 4,679,073, U.S. application Ser. No. 614,239 etc., a method of light regulation for such image reading apparatus, in which a standard white board as the standard for the white level and is disposed outside an original reading area, and in which regulation of the power supply to the light source is carried out in such a manner that the signal read from said white board constitutes a reference white signal.
The output signal of a CCD image sensor, obtained by reading an image of uniform density, may not be uniform, due to uneven luminocity of the light source, transmission characteristic of the lens or uneven sensitivity of the CCD image sensor. Such a phenomenon is called shading.
The assignee of the present invention has also proposed in the above-mentioned U.S. application Ser. No. 614,239, U.S. Pat. No. 4,675,533, U.S. application Ser. No. 706,179 etc., to correct such shading by electrically correcting the image signal obtained from the original image, based on an output obtained by reading a standard white board of a uniform density with the CCD image sensor.
In such an apparatus having the light regulating function and the shading correcting function, a standard white board is often used for both functions.
However such a standard white board is inevitably expensive since the white level thereof, particularly that of a reflective density lower than 0.1, is difficult to maintain in manufacturing. Besides, even if such a standard white board is incorporated in an apparatus with a well controlled white level, it becomes impossible to assure the white level of the output image and to achieve optimum light regulation and shading correction, due to discoloration or smear caused by temperature, humidity, dust contained in the cooling air, and infrared or ultraviolet light from the illuminating system.